Low cost fiber optic circulator

ABSTRACT

Improved optical devices, systems, and methods for selectively directing optical signals generally based on a Mach-Zehnder interferometer. Through accurate control of the phase relationship, the Mach-Zehnder interferometer allows optical signals to either be cumulatively combined so as to enhance the transmitted signal strength, or destructively combined so as to effectively prevent transmission from an optical signal port. This phase relationship can be controlled using a nonreciprocal device having a pair of retarder plates disposed along one of the two legs of the Mach-Zehnder interferometer so as to provide an optical isolator or circulator.

This application is a division of Ser. No. 09/081,261 filed on May 19,1998, now U.S. Pat. No. 6,075,596.

BACKGROUND OF THE INVENTION

The present invention relates to optical devices and systems, and in aparticular embodiment, provides a Mach-Zehnder interferometer baseddevice which may be used as an optical circulator or isolator.

An optical isolator is a nonreciprocal device which allows the passageof light in only one direction. A signal transmitted in a forwarddirection through a first port of an optical isolator will be passed toa second optical port. However, optical signals traveling in a rearwarddirection through the second optical port are blocked by the opticalisolator from reaching the first port. Such optical isolators have founda wide variety of uses in optical systems, particularly those usingoptical fibers.

An optical circulator is a nonreciprocal optical device related to theoptical isolator. Optical circulators allow the passage of light from afirst port to a second port, as in an optical isolator. However, ratherthan simply blocking signals traveling in a reverse direction into thesecond port, such signals are instead transmitted to a third port. Anytwo consecutive ports of an optical circulator are, in effect, anoptical isolator since signals travel in only one direction between theports.

Circulators will generally have three or more ports. Light transmittedinto the first or second port of a three port circulator will bedirected to the next higher number port. In a closed circulator, lighttransmitted into the third (or other highest number port) is passed tothe first port. In an open three port circulator, light directed intothe third port will be blocked by the circulator without transmittingthe light to any other active port. Regardless, the function performedby the circulator is called a circulating operation.

Several types of optical circulators have been developed. The structureof a conventional optical circulator includes three basic components:polarization beam splitters (PBSs), nonreciprocal Faraday rotators, andhalf-wave plates. Each beam splitter typically includes at least oneoptical deflection element such as a prism. Assembly of theseconventional circulators is fairly difficult, so that the cost ofconventional circulators is quite high.

Much work has gone into improving the performance of opticalcirculators. While conventional circulators provide an isolation ofabout 30 dB, additional birefringent crystals may be employed to improveisolation to over 40 db. Exemplary birefringent enhanced opticalcirculators are commercially available from E-TEK DYNAMICS, INC. of SanJose, Calif., and related devices may also be available from NIPPONTELEGRAPH AND TELEPHONE CORPORATION of Japan, FDK AMERICA, INC., ofCalifornia, and other sources. Generally, circulators which include botha conventional polarization beam splitter and birefringent crystals willhave costs higher than those of a conventional circulator.

Optical circulators based on light path deflection of birefringentpolarizers have also been proposed and implemented. These birefringentpolarizer based structures have enhanced isolation performance, butoften at a substantially higher cost. Moreover, optical circulatorsbased on either polarization beam splitters or birefringent polarizersare susceptible to polarization mode dispersion (PMD) if there is a lackof symmetry between the optical paths of the separated beams. Suchpolarization mode dispersion can limit the signal transmission speed ofan optical network, while the symmetrical circulator structures proposedto date are often very difficult to align and/or include highlyspecialized optical elements. Once again, exemplary birefringentpolarizer based optical circulators are commercially available fromE-TEK DYNAMICS, while competing structures may be available from NIPPONTELEGRAPH AND TELEPHONE CORPORATION of Japan, JDS FITEL, INC., ofCanada, PHOTONIC TECHNOLOGIES of Australia, and others.

The incremental improvements in high performance circulators haveprovided a variety of options for applications requiring high isolationwith low insertion loss. Unfortunately, the cost of each circulatorstructure is often prohibitive for applications requiring numerouscirculators. Moreover, there are applications for the opticalcirculating operation which do not require the performance of thesecostly structures. For example, in fiber optic networks, relatively lowcost amplification is available to overcome a relatively large amount ofinsertion loss.

A recent paper published by T. Shintaku et al. of NTT OPTO-ELECTRONICSLABORATORIES of Japan, describes a waveguide polarization-independentoptical circulator based on a Mach-Zehnder interferometer. Thisstructure combines two 45° Faraday rotators and two half-wave plateswith a Mach-Zehnder interferometer structure. A Faraday rotator and ahalf-wave plate are aligned symmetrically along each leg of theinterferometer, and the resulting circulator is described as providingan isolation of between 14.1 and 23.7 dB with an insertion loss ofbetween 3.0 and 3.3 dB.

While the recently proposed Mach-Zehnder interferometer based opticalcirculator appears to provide a useful alternative to circulators basedon conventional polarization beam splitters, birefringent crystalenhanced polarization beam splitters, and birefringent crystalpolarizers, particularly when the cost of these structures is notjustified. Nonetheless, it would be desirable to provide still furtherimprovements in optical circulators, and in optical circulation methods.It would be particularly desirable to provide optical circulatorstructures having improved manufacturability and still lower cost, whilemaintaining acceptable isolation, insertion loss, polarization modedispersion, and polarization dependent loss characteristics. It wouldfurther be desirable if these improvements were applicable to fiberbased optical circulators, integrated optical element systems, table topoptical networks, optical isolators, and the like.

SUMMARY OF THE INVENTION

The present invention provides improved optical devices, systems, andmethods for selectively transmitting optical signals. The opticaldevices of the present invention are generally based on a Mach-Zehnderinterferometer. Through accurate control of the phase relationship, theMach-Zehnder interferometer allows optical signals to either beconstructively combined (so as to enhance the transmitted signalstrength), or destructively combined (so as to reduce or preventtransmission). Surprisingly, this beneficial phase relationship can becombined with a simple asymmetrical nonreciprocal structure positionedalong one of the two legs of the Mach-Zehnder interferometer. Generally,two retarder plates will be positioned along one leg, while a lightsensitive fiber disposed along the other leg can allow the optical pathlength to be adjusted so as to avoid polarization mode dispersion. Asaligning retarder plates relative to each other is significantly easierthan independently aligning each retarder plate within the surroundingMach-Zehnder structure, the present invention provides significantfabrication advantages over known Mach-Zehnder interferometer basedoptical circulators.

In a first aspect, the present invention provides an optical devicecomprising a first optical element in a first optical path of a firstoptical signal. The first element directs a portion of the first signalalong a first optical path leg, and a portion of the first signal alonga second optical path leg. A second optical element is optically coupledto the first and second legs. The second element constructively combinesthe first signal portions to transmit a first signal along a secondoptical path. The second element also directs a portion of a secondoptical signal from the second path along the first leg, and a portionof the second signal along the second leg. A Faraday rotator is disposedalong the first leg or the second leg. First and second retarder platesare disposed along the first leg. The retarder plates are arrangedrelative to each other such that the first element destructivelycombines the second signal portions to diminish transmission of thesecond signal along the first optical path.

The first and second retarder plates will generally comprise half-waveplates that are affixed together. The optical axes of the half-waveplates will generally be offset by 45°. Conveniently, the Faradayrotator may also be disposed along the first leg adjacent the half-waveplates. In the exemplary embodiment, an optical waveguide having anindex of refraction which varies with exposure to radiation (forexample, a light sensitive fiber of the type typically used forfabrication of Fiber Bragg Gratings) is included along the second leg toadjustably equalize the first and second leg path lengths.

The first and second optical elements generally separate signals evenly,even where the signals have varying wavelengths. Suitable opticalelements include wavelength neutral 50/50 beam splitters, but will moretypically comprise 3 dB fiber couplers or 3 dB integrated waveguidecouplers. As the retarder plates and Faraday rotator may be disposedalong a single leg, the optical device may comprise an integrated opticwaveguide along a contiguous substrate in which the second leg extendscontiguously along the substrate from the first coupler to the secondcoupler. Coupling efficiency may be enhanced by including collimatinglenses between the couplers and the retarder plates. While conventionalcollimating lenses (particularly GRIN lenses) may be used, the devicesof the present invention will preferably make use of a microlens formedfrom a roughly quarter-pitch length of a graded index optical fiber.

In another aspect, the present invention provides an optical circulatorcomprising a first optical signal port for introducing a first opticalsignal. A first 3 dB coupler is optically coupled to the first port. Thefirst coupler directs a portion of the first signal along a firstoptical waveguide leg, and a portion of the first signal along a secondoptical waveguide leg. The first and second legs have optical pathlengths that are equal. A second optical signal port is provided forintroducing a second optical signal. A second 3 dB coupler is opticallycoupled to the first leg, the second leg, and the second port. Thesecond coupler directs a portion of a second optical signal from thesecond port along the first leg, and a portion of the second signalalong the second leg. The second coupler cumulatively combines portionsof the first signal portion, and transmits a resulting first passedsignal to the second port. A third optical signal port is opticallycoupled to the first coupler. A 90° Faraday rotator is disposed alongthe first or second leg, while first and second half-wave plates aredisposed along the first leg. The half-wave plates are arranged relativeto each other such that the second signal portions are interferinglycombined by the first coupler to reduce the second signal at the firstport. The second signal portions are cumulatively combined by the firstcoupler to a resulting second passed signal portion at the third port.

In another aspect, the present invention provides a method forfabricating a Mach-Zehnder based optical device. The method comprisesaffixing a first retarder plate to a second retarder plate. The affixedretarder plates are inserted within a first optical path leg between afirst coupler and a second coupler. A second optical path leg alsocouples the first and second couplers in parallel with the first leg,such that first signal portions transmitted by the first leg and thesecond leg are cumulatively combined by the second coupler. Similarly,second signal portions transmitted by the first leg and the second legare interferingly combined by the first coupler.

In a preferred embodiment, a first sheet of retarder material is affixedto a second sheet of retarder material with a predetermined alignment.The affixed sheets are partitioned into a plurality of affixed retarderplates, so that many pairs of plates are aligned simultaneously.

In yet another aspect, the present invention provides a method forfabricating an optical device. The method comprises exposing a radiationsensitive optical waveguide to radiation. The radiation sensitiveoptical waveguide is disposed along a first optical path leg between afirst coupler and a second coupler, the first leg being parallel with asecond optical path leg. The first and second legs have first and secondoptical path lengths, respectively. The exposing step is performed so asto equalize the first and second optical path lengths to produce adesired power split between a pair of optical signal ports opticallycoupled to the second coupler.

In yet another aspect, the present invention provides an optical devicecomprising a single mode optical fiber having an end. A graded indexfiber has first and second ends with a length of roughly one quarterpitch therebetween. The first end is adjacent to, and coaxially alignedwith, the end of the single mode fiber. Advantageously, this compactstructure can be used as a collimating/expanding lens, and/or to focuscollimate light signals into the single mode fiber.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an optical circulator according to the principles ofthe present invention.

FIG. 2 schematically illustrates the circulation function.

FIGS. 3A and 3B illustrate a Mach-Zehnder interferometer having at leastone 3 dB coupler.

FIG. 4 schematically illustrates how an optical fiber Mach-Zehnderinterferometer cumulatively combines optical signal portions in oneoutput port, and interferingly combines optical signals so as tominimize transmission of the signal to another output port.

FIG. 5 schematically illustrates a theoretical nonreciprocal structure,as can be used for calculating the desired transformations of theoptical signals along the first and second legs of the circulator ofFIG. 1.

FIG. 6 illustrates an alternative nonreciprocal Mach-Zehnderinterferometer structure for use as an optical circulator.

FIG. 6A schematically illustrates a method for pre-aligning a pluralityof half-wave plate pairs.

FIGS. 7A and 7B graphically illustrate how the optical components of thecirculator of FIG. 1 transform the optical signal as the light signalstravel from the left to the right (in FIG. 7A), and from the right tothe left (in FIG. 7B).

FIGS. 8A and 8B illustrate alternative circulator structures formed atleast in-part as an integrated optic waveguide.

FIG. 9 illustrates a still further alternative circulator according tothe principles of the present invention.

FIG. 10 illustrates the use of collimating lenses to enhancetransmission efficiency through the Faraday rotator and/or half-waveplates.

FIGS 11A-C illustrate alternative methods and structures for splicingoptical fibers so as to form the circulator of FIG. 1, which splicingmethods are particularly useful for forming a microlens collimator atthe end of a single mode fiber by attaching roughly a quarter-pitchlength of graded index fiber.

FIG. 12 schematically illustrates a method for adjusting an optical pathlength by exposing a light sensitive optical fiber to deep UV light.

DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENTS

Referring now to FIG. 1, a circulator 10 includes first and secondcouplers 12, 14. A first optical signal path leg 16 and a second opticalsignal path leg 18 extend in parallel between the couplers 12, 14. Fouroptical signal ports, here numbered 1, 2, 3, and 4 are available forintroducing optical signals into circulator 10, and for transmittingoptical signals passed by the circulator. Depending on the structure andarrangement of the circulator, these ports may comprise lengths ofoptical fiber, optical connections within an integrated opticalwaveguide, openings for sending and receiving optical signals, and thelike.

In the exemplary embodiment, first leg 16 of circulator 10 includes botha 90° Faraday rotator 20, and first and second half-wave plates 22, 24.The function and alignment of these structures will be described in moredetail hereinbelow. Second leg 18 of circulator 10 includes a lightsensitive fiber 26. This provides a simple and effective mechanism foradjusting an optical path length of second leg 18 relative to first leg16.

The function of circulator 10 can be understood with reference to FIG.2. Optical signals which are input into circulator 10 at port 1 will betransmitted to port 2, but will not be passed onto port 3. Signals inputinto the circulator at port 2 will pass onto port 3, but will not bedirected to port 1.

If isolator 10 of FIG. 2 were a closed three port circulator, opticalsignals input into the circulator at port 3 would then be transmitted toport 1. If, however, circulator 10 comprises an open circulatorstructure (for example, if port 4 is not connected to an output),optical signals input into port 3 will not be transmitted. Thecirculators of the present invention may be adaptable to varying numbersof ports, but are particularly well suited for use as open three portcirculators, or as closed four port circulators. It is also possible tomake use of these structures as an optical isolator, for example, bycoupling an input fiber to the first port 1 and an output fiber tosecond port 2.

As can be understood with reference to FIG. 3A, an optical signal 28 maybe separated at a Y-junction 30 so that a first portion of the signal 32is directed along the first leg 16, and so that an alternate signalportion 34 is directed along second leg 18. Assuming that signalportions 32, 34 have equal power, the amplitudes of the signals outputat ports 2 and 4 can be determined by the phase relationship of thesignals as they leave legs 16, 18. Where the optical path lengths of thetwo legs are such that optical signals 32, 34 are constructivelycombined and directed toward port 2, resulting signal 36 maytheoretically have the same power as input signal 28. In thisconfiguration, signal portions 32, 34 will be destructively combined bycoupler 14, such that the resulting signal at port 4 may be much lessthan the strength of input signal 28. In fact, where coupler 14 is ahighly accurate 3 dB coupler, and where legs 16, 18 have optical pathlengths such that signal portions 32, 34 are combined precisely 180° outof phase, the signal portions may substantially entirely be transmittedto port 2 and substantially isolated from port 4. Similarly, the powersplit between ports 2 and 4 of the Mach-Zehnder interferometerillustrated in FIG. 3B will depend on the phase relationship of signalportions 32, 34 as combined by second coupler 14.

To provide effective isolation between port 1 and port 4, coupler 12will split the power of input signal 28 evenly between legs 16, 18.Conveniently, couplers generally split and combine waves with a phasedifference of 90°. Because of this, signal portion 34 will also bedirected to each of ports 2, 4. Where legs 16, 18 are of equal length,signal portion 34 will be out of phase from signal portion 32 by π/2(90°). Similarly, due to second coupler 14, the portion of signalportion 34 which is directed towards port 4 will again be out of phaserelative to signal portion 34 by π/2. Hence, where legs 16, 18 are ofequal length, the signal portion transmitted from second leg 18 to port4 is out of phase by a total of π (180°) relative to the portion of thesignal which is transmitted across the upper part of the Mach-Zehnderinterferometer without transitioning through couplers 12, 14. Wherefirst and second couplers 12, 14 are very precise 3 dB couplers, half ofthe signal strength from first signal portion 32 will be directed towardeach of ports 2, 4. If the power split at each coupler is precisely 50%,and if the signals combined by coupler 14 and directed along port 4 are180° out of phase, no signal is present at port 4. Ignoring insertionlosses, the full strength of the signal will be transmitted to port 4.

In contrast, any part of the signal which is directed towards port 2 hasbeen transferred across coupler 12 or 14 a single time. As a result, thesignal portions provided to port 2 from signal portions 32 and 34 willbe in phase with each other, and will be cumulatively combined by secondcoupler 14. This results in transmission of signals from port 1 to port2, without transmitting the signal to port 4. It should be recognized,however, that the circulators of the present invention will oftenexhibit insertion losses of about 3 dB or more, due to transmissionefficiencies, adjustment errors, manufacturing tolerances, and the like.It should also be noted that the structure illustrated in FIG. 4 is areciprocal Mach-Zehnder interferometer: signals will pass from port 1 toport 2, and from port 3 to port 4, but will also be transmitted fromport 2 to port 1, and from port 4 to port 3.

To modify the Mach-Zehnder interferometers of FIGS. 3B and 4 to providea nonreciprocal circulating function, we modify our Mach-Zehnderinterferometer by introducing a structure into first leg 16 so as toperform first function ψ1 upon signal portion 32. Similarly, weintroduce a structure into second leg 18 which performs a secondfunction ψ2 on signal portion 34. We can provide our desirednonreciprocal results by setting:

ψ1−ψ2=2nπ, n=0,±1,±2, . . .

for signals traveling from left to right (that is, signals input atports 1 or 3), and by setting:

ψ1−ψ2=2mπ+π,m =0,±1,±2, . . .

for signals traveling from right to left (those input at ports 2 or 4),or vice-versa.

One symmetrical arrangement of components which fulfills these equationsis described in a paper entitled Waveguide Polarization-IndependentOptical Circulator Using A Mach-Zehnder interferometer, T. Shintaku etal. ECOC Publication No. 448, Sep. 22-25, 1997, the full disclosure ofwhich is incorporated herein by reference. This paper describes the useof a half-wave plate and a 45° Faraday rotator symmetrically disposedalong each of the first and second legs of a Mach-Zehnderinterferometer. To avoid the necessity of accurately aligning theoptical axis of the half-wave plates along each leg of theinterferometer, the present invention instead includes an asymmetricalnon-reciprocal structure comprising first and second half-wave plates22, 24 positioned along first leg 16, while a 90° Faraday rotator mayoptionally be disposed along first leg 16 or second leg 18. Positioningboth half-wave plates along the same leg significantly enhances themanufacturability of the circulators of the present invention. This isbecause two large sheets 33 of the half-wave plate material may beaccurately aligned relative to each other and bonded together asillustrated in FIG. 6A. These large, bonded sheets may then bepartitioned into a large number of accurately aligned and bonded pairsof half-wave plates. These bonded and aligned half-wave plates can thenbe aligned within the circulator structure illustrated in FIGS. 1 and 6in a single alignment step. Fabrication and assembly may be even furthersimplified by first assembling the half-wave plates and the Faradayrotator, and then positioning this sub-assembly along first leg 16, ascan be understood with reference to FIGS. 1 and 7A-9.

The polarization transformations effected by the half-wave plates andFaraday rotator are illustrated in FIGS. 7A and 7B. It should beunderstood that the structures imposing transformations Ψ1 and Ψ2 arehere assumed to not impose any other changes except the desired phasedifference (πor 180°) in one direction. Throughout thesetransformations, it is assumed that the observer faces the wave vectorof the transmitted light. Hence, clockwise angle rotations will beconsidered counter-clockwise angle rotations when traveling in thereverse direction.

Now reviewing the Jones transformation matrices as a signal moves fromleft to right along first leg 16 of circulator 10, we find that:$\begin{matrix}{\begin{bmatrix}E_{x}^{\prime} \\E_{y}^{\prime}\end{bmatrix} = \quad \begin{bmatrix}{{{\cos \quad \frac{\delta_{2}}{2}} + {j\quad \cos \quad 2\quad \theta_{2}\sin \quad \frac{\delta_{2}}{2}}},} & {j\quad \sin \quad 2\quad \theta_{2}\sin \quad \frac{\delta_{2}}{2}} \\{{j\quad \sin \quad 2\quad \theta_{2}\sin \quad \frac{\delta_{2}}{2}},} & {{\cos \quad \frac{\delta_{2}}{2}} - {j\quad \cos \quad 2\quad \theta_{2}\sin \quad \frac{\delta_{2}}{2}}}\end{bmatrix}} \\{\quad \begin{bmatrix}{{{\cos \quad \frac{\delta_{1}}{2}} + {j\quad \cos \quad 2\quad \theta_{1}\sin \quad \frac{\delta_{1}}{2}}},} & {j\quad \sin \quad 2\quad \theta_{1}\sin \quad \frac{\delta_{1}}{2}} \\{{j\quad \sin \quad 2\quad \theta_{1}\sin \quad \frac{\delta_{1}}{2}},} & {{\cos \quad \frac{\delta_{1}}{2}} - {j\quad \cos \quad 2\quad \theta_{1}\sin \quad \frac{\delta_{1}}{2}}}\end{bmatrix}} \\{\quad {\begin{bmatrix}{\cos \quad \Psi} & {{- \sin}\quad \Psi} \\{\sin \quad \Psi} & {\cos \quad \Psi}\end{bmatrix}\begin{bmatrix}E_{x} \\E_{y}\end{bmatrix}}}\end{matrix}$

while the portion of the signal traveling along second leg 18 may beanalyzed using the identity matrix: $\begin{bmatrix}E_{x}^{\prime} \\E_{y}^{\prime}\end{bmatrix} = {\begin{bmatrix}1 & 0 \\0 & 1\end{bmatrix}\begin{bmatrix}E_{x} \\E_{y}\end{bmatrix}}$

in which Ψ designates the rotation angle of Faraday rotator 20 (often90°), θ₁ designates the orientation angle of the fast axis of the firstwaveplate 22, 67 ₁ designates the phase retardation of the firstwaveplate 22, θ₂ designates the fast axis angle of the second waveplate24, and θ₂ designates the retardation of the second waveplate 24. E_(x)and E_(y) are electric field amplitudes along X and Y axes at the leftside of legs 16 and 18, respectively, while E′_(x) and E′_(y) correspondto the electric field amplitudes at the fight side of legs 16, 18. Byinserting our nominal values into the above equations Ψ=π/2, θ₁=π/4,δ₁=π,θ₂=0, and δ₂=π), the transformation matrix for first leg 16 for asignal traveling between port 1 and port 2 becomes: $\begin{bmatrix}E_{x}^{\prime} \\E_{y}^{\prime}\end{bmatrix} = {{{{\begin{bmatrix}j & 0 \\0 & {- j}\end{bmatrix}\begin{bmatrix}0 & {- j} \\{- j} & 0\end{bmatrix}}\begin{bmatrix}0 & {- 1} \\1 & 0\end{bmatrix}}\begin{bmatrix}E_{x} \\E_{y}\end{bmatrix}} = {\begin{bmatrix}1 & 0 \\0 & 1\end{bmatrix}\begin{bmatrix}E_{x} \\E_{y}\end{bmatrix}}}$

Using a similar analysis for a signal portion passing from port 2 toport 3 (as illustrated in FIG. 7B) along first leg 16, our Jones matrixis: $\begin{matrix}{\begin{bmatrix}E_{x}^{\prime} \\E_{y}^{\prime}\end{bmatrix} = \quad \begin{bmatrix}{\cos \quad \Psi} & {\sin \quad \Psi} \\{{- \sin}\quad \Psi} & {\cos \quad \Psi}\end{bmatrix}} \\{\quad \begin{bmatrix}{{{\cos \quad \frac{\delta_{1}}{2}} + {j\quad \cos \quad 2\quad \theta_{1}\sin \quad \frac{\delta_{1}}{2}}},} & {j\quad \sin \quad 2\quad \theta_{1}\sin \quad \frac{\delta_{1}}{2}} \\{{j\quad \sin \quad 2\quad \theta_{1}\sin \quad \frac{\delta_{1}}{2}},} & {{\cos \quad \frac{\delta_{1}}{2}} - {j\quad \cos \quad 2\quad \theta_{1}\sin \quad \frac{\delta_{1}}{2}}}\end{bmatrix}} \\{\quad {\begin{bmatrix}{{{\cos \quad \frac{\delta_{2}}{2}} + {j\quad \cos \quad 2\quad \theta_{2}\sin \quad \frac{\delta_{2}}{2}}},} & {j\quad \sin \quad 2\quad \theta_{2}\sin \quad \frac{\delta_{2}}{2}} \\{{j\quad \sin \quad 2\quad \theta_{2}\sin \quad \frac{\delta_{2}}{2}},} & {{\cos \quad \frac{\delta_{2}}{2}} - {j\quad \cos \quad 2\quad \theta_{2}\sin \quad \frac{\delta_{2}}{2}}}\end{bmatrix}\begin{bmatrix}E_{x} \\E_{y}\end{bmatrix}}}\end{matrix}$

while the signal portion transmitted along second leg 18 is again:$\begin{bmatrix}E_{x}^{\prime} \\E_{y}^{\prime}\end{bmatrix} = {\begin{bmatrix}1 & 0 \\0 & 1\end{bmatrix}\begin{bmatrix}E_{x} \\E_{y}\end{bmatrix}}$

Per our direction and angle convention, our nominal values are hereΨ=π/2,θ₁=π/4 δ₁=π, θ₂=0, and δ₂=π. These nominal values will result in afirst leg transformation matrix as follows: $\begin{bmatrix}E_{x}^{\prime} \\E_{y}^{\prime}\end{bmatrix} = {{{{\begin{bmatrix}0 & 1 \\{- 1} & 0\end{bmatrix}\begin{bmatrix}0 & j \\j & 0\end{bmatrix}}\begin{bmatrix}j & 0 \\0 & {- j}\end{bmatrix}}\begin{bmatrix}E_{x} \\E_{y}\end{bmatrix}} = {\begin{bmatrix}{- 1} & 0 \\0 & {- 1}\end{bmatrix}\begin{bmatrix}E_{x} \\E_{y}\end{bmatrix}}}$

From the above, it can be seen that light propagating from port 1 toport 2 undergoes the same polarization transformation regardless ofwhether it travels along first leg 16 or second leg 18. In contrast,light propagating from port 2 to port 1 will undergo a reciprocalpolarization transformations as it travels along first leg 16 ascompared to second leg 18. As a result, the light signal portionsdirected by first coupler 12 towards port 1 cancel each other out, whilethe signal portions directed by coupler 12 towards port 3 arecumulatively combined.

To provide broad band performance, optical paths 16, 18 should haveoptical path lengths that are as close as possible. Optical length hererefers to the index of refraction times the distance the light travels.Assuming the optical path length of first arm 16 is L₁ and that opticalpath length of second arm 18 is L₂, where circulator 10 functionsproperly for light having a wavelength of λ₁, we know that L₁−L₂=k x λ₁,where k = an integer. To ensure that this circulator will still functionproperly when the wavelength changes from 80 ₁ to 80 ₁ =Δλ, then weshould have:${\frac{\Delta \quad L}{\lambda \quad l} - \frac{\Delta \quad L}{{\lambda \quad l} + {\Delta \quad \lambda}}}\frac{l}{2}$

in which ΔL is equal to the difference in optical path length betweenfirst leg 16 and second leg 18. This leads to:${\Delta \quad L}\frac{\lambda \quad {l\left( {{\lambda \quad l} + {\Delta \quad \lambda}} \right)}}{2\Delta \quad \lambda}$

If we assume that λ₁,=1,500 nm, and that Δλ=40 nm, we find that ΔLshould be substantially less than 30 μm. This is a stringent requirementwhen splicing two 3 dB couplers together to form a Mach-Zehnderinterferometer. Stated differently, if we make use of signals having awavelength of 1,500 nm and allow the legs of our interferometer todiffer by as much as 3 mm, this will result in a Δλ which is much lessthan 0.4 nm. As optical signals are often significantly broader thanthis, isolation might well prove to be inadequate unless sufficient careis taken to ensure our optical path lengths are close to equal.

Further complicating our analysis on optical path lengths, the half-waveplates and Faraday rotators will generally impose a certain amount ofphase delay, in addition to the effects described by their Jonesmatrices. The phase delay imposed by a half-wave plate is generally:

π(n_(e)+n_(O))d/λ

in which n_(e) and n_(o) are the indices of refraction for theextraordinary and ordinary rays, while d is the thickness of theretarder plate. The phase delay imposed by a Faraday rotator will be

2,πnd/π

in which n is the index of refraction of the rotator material. Hence, ifwe are to obtain equal optical path lengths for the first and secondlegs of our interferometer, some compensation and/or adjustmentstructure should be provided.

FIG. 12 schematically illustrates a particularly advantageous method foradjusting the optic path length so as to enhance the bandwidth of thecirculators of the present invention. By fabricating second path 18 atleast in-part from an optic waveguide having an index of refractionwhich changes (usually increasing) when exposed to a radiation (such asdeep ultraviolet light 46), the optical path length of second leg 18 maybe selectively increased after fabrication of the circulator isotherwise complete. As the optical components present on the first legwill tend to give it the longer path length, adjustment mechanism 26need only be provided on second leg 18.

A variety of alternative radiation sensitive waveguide structures mightbe used. In fact, common optical fibers will gradually change in indexof refraction when exposed to deep ultraviolet light. However, to allowadjustments in a timely and reliable fashion, light sensitive fiberssuch as those used to fabricate Fiber Bragg Gratings may be included insecond leg 18. Such fibers are commercially available from 3M, SpecialtyFiber Division. These fibers may be adjusted by selective exposure todeep ultraviolet light, such as light having a wavelength of 193 nm or248 nm.

Since legs 16, 18 are equal in length, the primary source forpolarization mode dispersion will be the birefringent effects of thehalf-wave plates (the birefringent effects of the fiber can generally beneglected for this short path length). Assuming the half-wave platescomprise quartz, we can calculate the polarization mode dispersion asfollows: the difference in index between the ordinary and extraordinaryrays in quartz (at a wavelength of 1,550 nm) is about 0.009. Thethickness of a low order half-wave plate is about 0.5 mm. As a result,the optical path difference for our two polarization components will beabout 0.009 mm, or about 3×10⁻¹⁴ secs. Hence, polarization modedispersion does not appear to be significant.

The polarization dependent loss of the circulator structure is primarilycaused by the couplers, the polarization mode coupling of the fibers,and the half-wave plate tolerances. The polarization dependent loss of 3dB couplers can generally be limited to about 0.1 dB. Polarization modecoupling of fibers having short lengths should be negligible. As itshould be possible to limit polarization dependent loss resulting fromwave-plate tolerances, the total polarization dependent loss of thecirculator should be substantially the same as that of the 3 dBcouplers.

The isolation performance of the circulator of FIG. 1 will depend on avariety of factors. Any deviation from an even power split ratio willresult in leakage. Differences in the optical path lengths of the twoarms can decrease isolation, as can an error in the rotation angle ofthe Faraday rotator. Similarly, any retardation error in the half-waveplates, or any alignment error in the off-set angle between the twowave-plates will also degrade isolation performance.

A still further potential source of error in the circulator of FIG. 1 isthe thermal stability of the device. As described above, signal portionstraveling along second leg 18 may travel along an optical fiber (orother waveguide), the entire distance between couplers 12, 14. However,first arm 16 will include Faraday rotator 20, and first and second waveplates 22, 24, and will also include collimating lenses as describedabove. Such collimating lenses are often aligned and affixed within astainless steel tube, and the separation between the adjacent ends ofthe collimators will typically be about 2.0 mm. Based on the thermalcoefficient of expansion for stainless steel, this could result in achange of phase as large as 0.026 π/degree K of temperature change.Given the phase sensitivity of the circulators of the present invention,this could limit the overall thermal stability of the circulator. Toaccurately maintain the circulation function, the circulator may be heldat a constant temperature to ensure optimal performance. Alternatively,more thermally stable support structures may be used, or some thermalcompensation structure may be included in either the first or secondleg. Such a compensation structure might be incorporated into the phaseadjustment mechanism in some embodiments.

A wide variety of materials and structures may be used in Faradayrotator 20. Some of the candidate materials are listed in the followingtable:

Material Operation Wavelength Magnet Required (BiYbTb)Fe₅O₁₂ 1.3-1.6 μmYes Cd_(1-x-y)Mn_(x)Hg_(y) 0.6-1.1 μm Yes (Typical) (X: 0.15-0.17) (y:0.12-0.13′) CdMnHgTe 0.6-1.1 μm Yes (TbBe)₃(FeGa)₅O₁₂ 1.5-1.7 μm No(GbBi)₃(FeAlGa)₅O₁₂ 1.3-1.6 μm Yes (TbBi)₃Fe₅O₁₂ 1.3-1.6 μm Yes(RBi)₃Fe₅O₁₂ 0.8 μm Yes (R: Rare Earth Element) YIG (Yttrium IronGarnet) 1.1-1.6 μm Yes TGG (Terbium Gallium 0.5-1.1 μm Yes Garnet)(BiTb)₃(FeGa)₅O₁₂ 1.3-1.6 μm Yes

A variety of suitable materials for fabrication of half-wave plates 22,24 are also known. Suitable retarder plates may comprise calcite,crystal and quartz, or the like.

FIGS. 8A, 8B, and 9 illustrate alternative circulator structuresaccording to the principles of the present invention. In the embodimentof FIG. 8A, first and second couplers 12, 14 each comprise an integratedoptical coupler chip 40. Second leg 18 comprises an optical fiber 42,while first leg 16 includes a combination of optical fiber, the opticalwaveguides of integrated optic chips 40, and the Faraday rotator andhalf-wave plates described hereinabove.

In the embodiment of FIG. 8B, an integrated optic waveguide 44 includesfirst and second couplers 12, 14 and a contiguous optic waveguide forsecond leg 18. Faraday rotator 20 and half-wave plates 22, 24 areinserted into a slot defined in the substrate of integrated opticwaveguide 44. This substrate will typically comprise SI, LiNbO₃,polymer, or the like. Optical fibers 42 may be coupled to integratedoptic waveguide 44 for transmission of the circulated optic signals, asdesired. In some embodiments, leg 18 may again be light sensitive so asto allow adjustment to the path length.

FIG. 12 schematically illustrates a particularly advantageous method foradjusting the optic path length so as to enhance the bandwidth of thecirculators of the present invention. By fabricating second path 18 atleast in-part from an optic waveguide which changes its index ofrefraction when exposed to a radiation such as deep ultraviolet light46, the optical path length of second leg may be selectively increasedafter fabrication of the circulator is otherwise complete. As theoptical components present on that first leg will tend to make thatoptical path leg longer, adjustment mechanism 26 need only be providedon second leg 18 where both the Faraday rotators and half-wave platesare disposed on the first leg.

A variety of alternative radiation sensitive waveguide structures mightbe used. In fact, common optical fibers will gradually change in indexof refraction when exposed to deep ultraviolet light. However, to allowadjustments in a timely and reliable fashion, light sensitive fiberssuch as those used to fabricate Fiber Bragg Gratings may be included insecond leg 18. Such fibers are commercially available from 3M, SpecialtyFiber Division. These fibers may be adjusted by selective exposure todeep ultraviolet light, such as light having a wavelength of 193 nm or248 nm.

A still further alternative circulator structure is illustrated in FIG.9. In this embodiment, 50/50 neutral splitters 48 provide the functionalequivalent of the couplers described above, while mirrors 50 direct thesignal portions along the first and second legs. Nonetheless, the Jonesmatrices and functional interactions of the optical components remainssubstantially as described above.

Referring now to FIG. 10, transmission of signal portion 32 along firstleg 16 may be enhanced by expanding and collimating the signal passingthrough Faraday rotator 20 and first and second half-wave plates 22, 24.In this embodiment, optical fibers 42 are held in ferrules 52, and theends of the fibers and ferrules are polished together at a slight angle(typically between about 8 and 12°). A quarter-pitch or nearquarter-pitch GRIN lens 54 is coaxially aligned with ferrule 52. The endof each GRIN lens is polished at a reciprocal angle to the adjacentferrule end, and the GRIN lens end and ferrule end are held in closeproximity, but with a slight gap therebetween. To minimize insertionlosses, all transmission surfaces will have anti-reflective coatings.

While theoretical calculations indicate that GRIN lenses of 0.25 pitchwould most efficiently expand and focus the transmitted signals,experience has shown that GRIN lenses having a pitch of about 0.23 willprovide better results. This may be due in-part to the fact that suchcalculations generally assume that optical fibers 42 transmit light as apoint source, while the light is actually dispersed (although over avery small cross-sectional area).

A variety of alternative collimating structures may be used in place ofstandard GRIN lenses 54. Optionally, GRIN lenses 54 may be replaced bycollimating microlenses, with the surrounding structure remainingsubstantially as described with reference to FIG. 10. Such microlensesare available from Corning Corp. as Asperic Lens #101. Once again, allsurfaces should have anti-reflective coatings to minimize insertionlosses.

Still further alternative collimating structures are possible. As can beunderstood with reference to the equations given above, it is generallybeneficial to minimize the total path length of both legs to enhance theoverall performance of the circulators. In fact, it is generallybeneficial to fabricate compact miniaturized structures for many opticalsystems. Toward that end, the present invention further provides a novelcollimating structure formed by axially aligning the optical fiber(which will typically comprise a single mode fiber) with a short lengthof graded index fiber.

Single mode optical fibers often have cores between about 2.0 and 10.0μm. In contrast, graded index fibers will often have cores as large asabout 50.0 μm. Nonetheless, the unjacketed diameters of these opticalfibers may be quite similar, typically being about 125 μm. Hence, byaligning and affixing a quarter-pitch (or roughly quarter-pitch) lengthof graded index fiber to a single mode fiber, the optical signal may beboth radially expanded and collimated. Similar results may be achieved(with an optionally longer axial length) by using a graded index fiberhaving a length of 0.25+n pitch, n being an integer (0, 1, 2, 3, . . .), n optionally being less than 1,000, often less than 100, and in somecases less than 10. The equivalent outer dimensions of the single modeand GRIN fibers greatly facilitates axially aligning these structures.To minimize the overall length of first and second legs 16, 18 of ouroptical circulator, and also to accurately affix an appropriate lengthof graded index fiber to our single mode fiber, it is advantageous tocouple our single mode and graded index fibers with an end-to-endattachment (rather than using fiber fusing or coupling techniques).Structures and methods for providing end-to-end coupling of opticalfibers can be understood with reference to FIGS. 11A-C.

In the splicing method and structure illustrated in FIG. 11A, opticalfiber 42 is spliced to fiber structure 56 by inserting the fiber in oneend of a glass capillary 58. A photosensitive adhesive 60 is disposedwithin glass capillary 58 adjacent the end of fiber 42, and fiberstructure 56 is inserted into the opposite end of glass capillary 58 sothat the ends of the fiber structure and optical fiber are in closeproximity. Ultraviolet light 62 is directed through glass capillary 58to photosensitive adhesive 60 so as to cure the adhesive, whichpreferably has an optical index matching that of the adjacent opticalfibers.

Where fiber structure 56 comprises a graded index optical fiber to beused as a collimating lens, the graded index fiber may later be brokenat the desired pitch length from its bonded end. Alternatively, fiberstructure 56 may be gradually polished to provide the proper pitchlength. It should be noted that the preferred graded index fiber lengthwill again not necessarily be exactly n+0.25 pitch. Possibly because asingle mode fiber has a significant cross-section relative to a gradedindex fiber, the pitch may be anywhere in a range from about n+0.20 toabout n+0.25, the graded index fiber ideally having a pitch of aboutn+0.25. The splice method illustrated in FIG. 11A (and those of FIGS.11B and C) may also be used to splice a length of photosensitive fiber26 so as along second leg 18 (as described with reference to FIGS. 1 and12), or to splice two fiber couplers together.

FIGS. 11B and 11C illustrate related methods for splicing optical fiber42 to any of a variety of fiber structures 56. In these embodiments, theoptical fibers are axially aligned using a V-groove plate 64. Bonding isagain provided by a photosensitive adhesive with optical index matching,and by curing the adhesive with ultraviolet light 62 while the fibersare held in alignment. Optionally, the bonded fibers may remain affixedto V-groove plate 64, or may instead by removed from the groove afterbonding is complete.

While the exemplary embodiments of the present invention have beendescribed in some detail, by way of illustration and for clarity ofunderstanding, a variety of alternatives, modifications, and changeswill be obvious to those of skill in the art. Hence, the scope of thepresent invention is limited solely by the appended claims.

What is claimed is:
 1. A method for fabricating an optical device, themethod comprising exposing a radiation sensitive optical wave guide toradiation, wherein the radiation sensitive optical wave guide isdisposed along a first optical path leg between a first coupler and asecond coupler, wherein a second optical path leg extends between thefirst and second couplers, the first and second legs having first andsecond optical path lengths, respectively, and wherein the exposing stepis performed so as to equalize the first and second optical path lengthsto produce a desired power split between a pair of optical signal portsoptically coupled to the second coupler.
 2. A method as claimed in claim1, wherein the exposing step compensates for optical elementsdistributed asymmetrically between the first and second legs, theoptical elements defining a non-reciprocal structure, the optical devicecomprising a circulator or isolator.
 3. A method for fabricating anoptical device, the method comprising exposing a radiation sensitiveoptical wave guide to radiation, wherein the radiation sensitive opticalwave guide is disposed along a first optical path leg between a firstcoupler and a second coupler, wherein a second optical path leg extendsbetween the first and second couplers, the first leg having a firstoptical path length, and wherein the exposing step is performed so as toalter the first optical path length and produce a desired power splitbetween a pair of optical signal ports optically coupled to the secondcoupler.